Modulation of the efficacy of synaptic transmission is a key component of neural plasticity, which is, in turn, a defining feature of the brain. Receptors located on the presynaptic terminal are known to profoundly depress neurotransmitter release. Of these, G protein-coupled receptors are ubiquitous. We have demonstrated that G protein-coupled receptors inhibit synaptic transmission by an effect mediated directly at the core complex for vesicle fusion. Gbg binds directly to SNAP-25 incorporated into the SNARE complex of primed vesicles. We now hypothesize that the mechanism for this modulation is an alteration in fusion pore dynamics as the synaptic vesicle releases transmitter into the synaptic cleft. This change in mode of transmission is likely to have profound consequences both to the way synaptic transmission actually works, and to the way we interpret it as working. Transient fusion, will conserve vesicles during repetitive activity, it may also change the mode of synaptic transmission by favoring different postsynaptic receptors with different affinities and different subsynaptic locations. In this proposal we seek to determine the mechanism by which presynaptic G protein-coupled receptors modulate release of neurotransmitter. We will quantify the duration of incomplete vesicle fusion events from the presynaptic terminal caused by presynaptic G proteins using styryl dye labeling of fusing synaptic vesicles? We also wish to determine the affects of presynaptic receptor activation on the timecourse and concentration of neurotransmitter in the synaptic cleft by measuring and modeling the kinetics of postsynaptic receptor activation. We will also determine the effects that altering these parameters will impose on the relative activation of different postsynaptic ionotropic receptors. Finally, we will investigate the affect of presynaptic G proteins on the properties of individual synaptic contacts using high-speed imaging techniques.